Render Result vs Viewer Node
The Composite Node(or the Render Result) outputs to the image editor and is your final render and is where you save from. The viewer node is used as a backdrop for the compositor. It's not intended for a final render but rather to preview the render result allowing you to more easily adjust compositor settings. 

Also in the compositor make sure you check Use Nodes and Backdrop. The backdrop is used to see what is inputted into the Viewer node.

Viewer Node: https://docs.blender.org/manual/ko/dev/compositing/types/output/viewer.html

Composite Node:
https://docs.blender.org/manual/ko/dev/compositing/types/output/composite.html